A Long-Lived Accretion Disk Around a Lithium-Depleted Binary T Tauri Star
    Russel White, Caltech
    Lynne Hillenbrand, Caltech

    We present a high dispersion optical spectrum of St 34 and identify the system to be a spectroscopic binary with components of similar luminosity and temperature (both M3).  Based on kinematics, signatures of accretion,and location on an H-R diagram, we conclude that St 34 is a classical T Tauri system belonging to the Taurus-Auriga T Association.  Surprisingly, however, neither component of the binary shows Li I 6708 A absorption, the most universally accepted criterion for establishing stellar youth.  In this uniquely known case, the accretion disk appears to have survived longer than the lithium depletion timescale.  Comparison with pre-main sequence evolutionary models imply for each component a mass of 0.37 Msun and an isochronal age of 9 Myr, which is much younger than the predicted lithium depletion timescale of ~25 Myr.  Although a distance closer than that of Taurus or a hotter temperature scale could reconcile this discrepancy, similar discrepancies in other systems suggest a possible problem with evolutionary models.  Regardless, with an age >10 Myr, St 34 is one of the oldest accreting T Tauri stars known.  The large ratio of non-accreting to accreting stars in clusters of age ~10 Myr suggest that St 34 may have many 10s of coeval co-members.  Those with early M spectral types should likewise be lithium-poor.  Current membership lists of Taurus may therefore be incomplete.  Finally, we suggest that the existence of long-lived accretion disks like that around St 34 and some stars in the TW Hydrae Association is caused by the presence of a sub-AU separation companion which delays disk dissipation by tidally inhibiting, though not preventing, circumstellar accretion.